tests: timer_api: Decrease tick rate for nRF RTC timer

The default tick rate for the nRF RTC timer is 32768 Hz, so one tick
is ~30 us. This turns out to be too little for the following loop
executed in user mode on the Network core in nRF5340:
```
	do {
		t0 = k_uptime_ticks();
		rem_ticks = k_timer_remaining_ticks(&remain_timer);
		t1 = k_uptime_ticks();
	} while (t0 != t1);
```
The time between the two calls to `k_uptime_tick()` is there always
above 30 us, so the loop never ends.

This patch decreases the tick rate for all nRF platforms because on
other nRf SoCs the time mentioned above is also close to 30 us and
apparently changes in code completely unrelated to this test affect
execution time of system calls in the above loop - the test started
to fail after commit 0014dd05 that
changes fdtable was merged and if `fdtable.c` is for example just
temporarily excluded from the build, the test passes.

The root cause of the problem seems to be related to user space
handling and this should to be investigated further. This patch
is applied only to allow this test to pass for the time being.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>